810 improve cmakelists

[shivendra-dev54]

Reason for this PR

This PR addresses #810 to improve the portability and installation of the GraphAr C++ library. The current setup makes it difficult for downstream projects to use find_package(graphar), which this update resolves by modernizing the CMake export logic.

What changes are included in this PR?

• Modern CMake Targets: Added support for the graphar::graphar namespace via install(EXPORT).
• Hybrid Linking Strategy: Updated dependencies (Arrow/Parquet) to PUBLIC visibility to ensure proper propagation to consumers.
• Relocatable Installation: - Integrated GNUInstallDirs for standard-compliant paths.
  • Set CMAKE_INSTALL_RPATH using $ORIGIN (Linux) and @loader_path (macOS) to ensure the library is relocatable.
• Config File Modernization: Updated graphar-config.in.cmake using @PACKAGE_INIT@ and find_dependency for robust downstream resolution.
• Backward Compatibility: Preserved the legacy GRAPHAR_LIBRARIES and GRAPHAR_INCLUDE_DIR variables.

Are these changes tested?

• Verified that all 20 existing C++ unit tests pass (ctest).
• Confirmed that make install generates a valid relocatable file structure.
• Successfully linked a separate "Hello World" project using find_package(graphar REQUIRED).

Are there any user-facing changes?

Yes. Users can now integrate GraphAr into their own projects using the standard CMake pattern:

find_package(graphar REQUIRED)
target_link_libraries(my_app PRIVATE graphar::graphar)